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Questions before I install...

Hello, I want to run a water line
from my house to my garden, where I want to install about 4-6 faucets to
connect drip irrigation to. I would like to be able to use the first
faucet in the line year-round because that one will service the
greenhouse.
What kind of cold weather precautions do I need
to take? (our frost line is 18 inches and we go in and and out of light
frosts all winter).
Can I bury it all below frost line and
set up a drain line for the first faucet (so it can drain water when not
being used). And then blow out or drain the other faucets during the
winter. Would burying below frost line help?
Do I need to insulate the pipe that is connecting to the house pipe above the frost line?
Is a drain line the best way to go for the faucet that I want to use year-round?
Should I put in a valve to blow out the other lines with a compressor or should I just install drain lines for them?
Do I need to set up a frost-free faucet on the one that I am using year-round? Sorry for all of the questions!!!!!

Unless the greenhouse is heated, your "first faucet" has to be treated as if it's out of doors. That means you have to trench about a foot deeper than the frost line (call before you dig, and get utility locates) from the house to the greenhouse, and use a "yard hydrant" as the greenhouse faucet. These hydrants are like giant frost-free faucets, made to connect to deeply buried waterlines, for year-round outdoor service. At or near the point where the yard hydrant is connected, you can tee off to a line to feed the warm-weather faucets, using a "curb-stop" valve that opens and closes with the same sort of key a city water department uses. Some of these curb stops have a drain feature that will empty the downstream line when the curb stop is closed, assuming you keep the waterline pitched so as to allow this drainage. Both curb stop and yard hydrant have a bed of gravel around them, to allow for better drainage, as the yard hydrant also will self-drain when it is shut off.

The supply for this project will start in the basement, exiting below grade. You might need a backflow preventer in the supply, because of the below-grade drains in the outdoor valves. The kind of pipe used for the deep outdoor line would be specified as 160-psi-rated poly pipe, preferably the medium density version favored by submersible pump installers, which is a bit softer and more easily clamped to the brass insert fittings a project like this will employ.

More Questions...

Thanks so much for your help! I have a few more questions... So, do I
need to make that t for the warm water faucets, below the frost line, or
can I make it anywhere? If I need to make it below the frost line, then
there is no way I can pitch the rest downhill, it will just be too
deep! Also, what kind of stores do you recommend going to, to purchase
supplies? Lowe's or Home Depot, or some other kind of irrigation
wholesaler? Thanks again for your efforts...

If all your outdoor faucets are yard hydrants ($$$) then you don't have to worry about pitching their supply line, as the line will be below the frost level at all points. The only water subject to freezing will be in the hydrants themselves, and that water drains out every time the hydrant is shut off. While this approach seems extravagant, it simplifies the design and saves you the expense of a curb-stop valve to feed seasonal faucets.

Your frost line is much shallower than mine, so different construction standards might apply. For deep installs, the high-reliability poly pipe and brass insert fittings are standard practice, since you never want to have to revisit work buried over four feet deep. You can find videos showing PVC pipe used for the supply for a lawn hydrant set about two feet deep.